The present invention relates generally to the field of motor drive systems utilized for driving office equipment and/or media feed systems; and, more specifically, to the field of micro-stepping motors and motor control torque resonance and acoustic noise reduction.
Though there are motors of varied kind, shape and configuration within the equipment that serves our daily lives (everything from office equipment to cameras), they all serve the basic purpose of converting electrical energy into mechanical energy to drive a system component. One segment of this diverse motor population is represented by the group referred to as xe2x80x9cstepper motorsxe2x80x9d. These motors convert a series of electrical pulses into rotational movements of specific duration. The movement resulting from each pulse is consistent both in time and space, thus making stepper motors an ideal means for positioning objects or media.
One example of the use of a stepper motor to accurately and consistently position an object is disclosed in U.S. Pat. No. 5,410,338 for a Method And Apparatus For Moving An Object With Uniform Motion, issued Apr. 25, 1995 to Jadrich et al. (hereinafter referred to as xe2x80x9cJadrichxe2x80x9d). Jadrich discloses an apparatus for printing an image to an object. The apparatus comprises a lead screw adapted to move the object to be printed upon and a stepper motor adapted to rotate the lead screw in a series of rotational steps wherein each step was less than that of the lead screw. Thus, Jadrich is an excellent example of a drive need that is successfully met by the use of a stepper motor. In this case, the lead screw, for each revolution, possessed a predetermined pattern of relationships between an amount of displacement of the object and each step of the stepper motor.
Another example of the use of stepper motors is provided by U.S. Pat. No. 5,852,354 for a Mutiplexing Rate Generator/Counter Circuit For Controlling Stepper Motors issued Dec. 22, 1998 to J. Randolph Andrews (hereinafter referred to as xe2x80x9cAndrewsxe2x80x9d). In Andrews, a multiplexing circuit was provided so that up to sixteen different stepping motors could share a single rate generator.
As the stepper motor population group diversified with the myriad applications that the motor population could serve, stepper motors were modified and redefined through control of the circuits that guide the electrical impulses that drive the motors. One of the early problems that led to such modification was the determination that when the motor was running at conventional full or half-step mode, there was a significant increase in noise level and torque resonance. The answer to the problem was found in xe2x80x9cmicro-steppingxe2x80x9d the motor.
The increase in noise and torque resonance are a byproduct of the conventional stepper motor design. Permanent magnet type stepper motors generally comprise coil windings, magnetically conductive stators and a permanent magnet rotor. An electromagnetic field having opposite poles (a north and a south pole) is created when the coil winding is energized. The magnetic field is carried by the magnetically conductive stators, thus causing the rotor to align itself with the magnetic field. The field, in turn, can be shifted by xe2x80x9csteppingxe2x80x9d (through sequentially energizing) the stator coils to generate a rotary motion. Various forms of stepping exist; these include: xe2x80x9cone phase onxe2x80x9d and xe2x80x9ctwo phase onxe2x80x9d stepping, as well as half-stepping.
Half-stepping occurs when an xe2x80x9coffxe2x80x9d state is inserted between transitioning phases, thus cutting the stepper motor""s full step angle in half. Half stepping, however, results in a loss of torque compared to other forms of stepping such as xe2x80x9ctwo phase onxe2x80x9d. The loss of torque is the result of one of the coil windings not being energized during alternate half steps, thus reducing the electromagnetic force being exerted on the rotor.
Coil windings too, can be of varied type such as xe2x80x9cbipolarxe2x80x9d or xe2x80x9cunipolarxe2x80x9d winding. In bipolar coil winding, each phase consists of a single winding. If the current flow in the windings is reversed through switching, then the electromagnetic polarity of the phase is reversed. Unipolar winding, on the other hand, consists of two windings on a common pole. The opposite poles are created when the separate windings are energized. In unipolar winding, the electromagnetic polarity from the drive to the coil windings is not reversed as is the case with bipolar windings. Unipolar windings though, produce less torque than their bipolar counterparts because the energized coil only utilizes half as much of the conductive winding (typically copper).
In addition to problems with torque in its many forms, stepper motors generate a resonant frequency as a result of their basic spring-mass configurations. When the motor""s step rate equals the motor""s natural frequency, there is an increase in the noise level of the motor as well as an increase in motor vibration. In addition to the increased noise burden on system users, continued vibration can weaken the system structure and efficiency.
Changing the step rate or microstepping the motor are the two most common means of reducing resonance problems. Microstepping divides a full step into smaller steps and helps reduce noise levels and produce smoother output motion in addition to reducing resonance.
The microstepping of a motor has been disclosed in the prior art with reference to the addressing of specific needs. Such a need is answered in U.S. Pat. No. 5,359,271 for a Microstepping Bipolar Stepping Motor Controller For Document Positioning issued Oct. 25, 1994 to Frederick K. Husher (hereinafter referred to as xe2x80x9cHushesxe2x80x9d). In Husher, one object of the invention was to provide an inexpensive controller that could drive a stepping motor in a microstep mode while yielding a multiple of defined position steps as compared to the number of physical motor poles.
Another need answered by microstepping a motor is disclosed in U.S. Pat. No. 4,710,691 for a Process And Apparatus For Characterizing And Controlling A Synchronous Motor In Microstepper Mode issued Dec. 1, 1987 to Bergstrom et al. (hereinafter referred to as xe2x80x9cBergstromxe2x80x9d). Bergstrom discloses the use of motor characterization stored in memory for use as a control of the rotational motion of the stepper motor.
The prior art, in addressing such issues as resonance and motion control, has still failed to produce a relatively simple drive system that can be easily adapted to a variety of equipment in a simple, yet efficient, way. There exists a need for a low-cost, easily replicable, micro-stepping motor in which the torque resonance and noise levels are reduced over those of prior art motors.
In the present invention, in a low-cost stepper motor drive system is provided.
The drive system comprises a micro-stepping motor and a logic circuit. The unique configuration of the circuit logic circuit provides a means of addressing the needs of the art field. The logic circuit includes two integrated circuits for controlling one or more phases of the micro-stepping motor. Each one of the integrated circuits is a motor driver capable of producing up to sixteen micro-steps per full step. Further, each one of the integrated circuits takes a set of analog/digital bits and a phase bit as input.
Additionally, within the logic circuit, there is included a micro-controller for controlling the micro-stepping motor. The micro-controller further comprises six control bits, which direct the controller in outputting a set of step sequences to the integrated circuits. The control bits further comprise a direction bit, a mode bit and a profile/index bit. The direction bit directs the micro-stepping motor to run in a clockwise or a counterclockwise direction as determined by a system configuration. The mode bit sets the micro-stepping motor to run in xe2x85x9th or {fraction (1/16)}th step. The micro-controller can run in one of two program modes as selected by the profile/index bit, the program modes further comprise a stand-alone mode and a pulse controlled mode.
The stand-alone mode utilizes two profile bits to select one of a set of four slew velocities. The stand-alone mode will accelerate the micro-stepping motor then slew at a preprogrammed velocity as selected by the two profile bits.
In the pulse controlled mode, however, the motor velocity of the micro-stepping motor is directly related to an input pulse signal wherein each time the micro-controller receives the input pulse signal the micro-controller will output a step to the integrated circuits.
The logic circuit further includes a driver circuit for driving current through each one of the motor phases, and two snubber circuits operatively located between the outputs of each one of the motor phases of each integrated circuit. The snubber circuits serve the purpose of stabilizing the voltage level of the logic circuit. Each one of the snubber circuits further comprises a resistor and a capacitor.